A shape of a conventional grommet is disclosed, for example, in Japanese Patent Laying-Open No. 2006-188964 (Patent Document 1). Japanese Patent Laying-Open No. 2006-188964 (Patent Document 1) discloses an example of an attachment structure for attaching an engine cover covering an engine to an engine component, in which the diameter of an attachment part formed in the engine member and the diameter of a hole formed in a tubular grommet are defined thereby improving the efficiency of the attaching operation.    Patent Document 1: Japanese Patent Laying-Open No. 2006-188964